1. Field of The Invention
The present invention relates generally to an apparatus and method for self-diagnosing a fuel supply control system applicable to an internal combustion engine, and, more particularly, relates to the self-diagnosing apparatus and method for the system of the fuel supply quantity control system having an air/fuel mixture ratio feedback control function in which with an air/fuel mixture ratio controlled state correlated to an exhaust gas characteritic taken into account, the diagnostic operation for the fuel supply control system can be carried out.
2. Description of The Background Art
A U.S. Pat. No. 4,924,836 exemplifies an air/fuel mixture ratio learning and controlling system having an air/fuel mixture ratio feedback correction control function and applicable to an electronically controlled fuel injection system of an internal combustion engine.
In addition, a U.S. patent application Ser. No. filed on Apr. 16, 1992 titled SYSTEM AMD METHOD FOR LEARNING AND CONTROLLING AIR/FUEL MIXTURE RATIO FOR INTERNAL COMBUSTION ENGINE also exemplifies the similar system and method for learning and controlling the air/fuel mixture ratio.
In such previously proposed air/fuel mixture ratio learning and controlling systems, an air/fuel mixture ratio feedback correction control is carried out such that a basic fuel injection quantity Tp calculated from two parameters indicating an engine during condition (for example, an intake air quantity Q and engine revolution speed N) related to sucked air quantity is corrected with an air/fuel mixture ratio feedback correction coefficient LMD set through a proportion-integration control of the air/fuel mixture ratio on the basis of rich or lean determination of the actual air/fuel mixture ratio by means of an oxygen concentration sensor installed in an exhaust system of the engine with respect to a target air/fuel mixture ratio (for example, a stoichiometric air/fuel mixture ratio) so that the actual air/fuel mixture ratio is feedback-controlled so as to be matched with the target air/fuel mixture ratio.
A deviation of the air/fuel mixture ratio feedback correction coefficient LMD from a reference value (target convergence value) is learned for the respective driving regions defined according to the intake air quantity Q and engine revolution speed N, the whole engine driving region being divided into a plurality of the driving regions, so as to derive a learning correction coefficient KBLRC at the corresponding driving region. The basic fuel injection quantity Tp is corrected with the learning correction coefficient KBLRC so that a base (or bare) air/fuel mixture ratio derived without the feedback correction coefficient LMD is substantially coincident with the target air/fuel mixture ratio. During the air/fuel mixture ratio feedback control mode, the basic fuel injection quantity Tp is further corrected with the feedback correction coeffcient LMD thereby to calculate a final fuel injection quantity Ti.
The correction of the basic fuel injection quantity Tp corresponding to a required value of the air/fuel mixture ratio different for each engine driving condition can be carried out and the air/fuel mixture ratio feedback correction coefficient LMD becomes stable in the vicinity to the reference value so that an air/fuel mixture ratio controlability can be improved.
On the other hand, when, in such previously proposed fuel injection quantity controlling systems as described above, failures, deteriorations, and/or variations in the charachateristics of products during manufactures thereof, i.e., components in the fuel supply control systems such as fuel injection valve(s), fuel pump, and airflow meter to detect the intake air quantity are generated, the base air/fuel mixture ratio is usually deviated from the target air/fuel mixture ratio so that harmful components such as CO, HC, and NOx included in the exhaust gas are increased.
A self-diagnosing method for monitoring a worsening of the exhaust gas characteristic due to the deviation of the base air/fuel mixture ratio from the target air/fuel mixture ratio may include a step of deriving an average value of the learning correction coefficient KBLRC (which is the result of learning of the requested correction value to achieve the target air/fuel mixture ratio for the respective driving regions) and a step of determining that the learning correction level becomes large due to occurrence of some abnormality in the fuel supply controlling system when the average value of KBLRC is deviated by a predetermined value from an initial value of KBLRC.
As appreciated from FIGS. 13 through 16, however, although the deviation of the average base air/fuel mixture ratio has the same level, a change (or gradient) according to the driving condition in the air/fuel mixture ratio deviation characteristic representing the base air/fuel mixture ratio devation which is varied (or has a gradient) according to the engine driving condition (for example, intake air quantity) causes the characteristic of exhaust gas to be differed according to the driving condition. Consequently, many cases occur in which the exhaust gas characteristic (percentage of each of all harmful components falls in a statutory limit value (as shown in FIG. 3, no change in the error rate on the deviation in the air/fuel mixture ratio occurs) and in which the percentage of either only NOx or only CO and HC exceeds the statutory limit value (as shown in FIGS. 14 or 15, the change in the error rate depending on the driving condition occurs).
It is, therefore, difficult to determine accurately a state of the deviation of the air/fuel mixture ratio resulting in the worsening in the exhaust gas characteristic from the average value of the learning correction coefficient KBLRC indicating the level of the average base air/fuel mixture ratio deviation.
In addition, in such a situation that the correction of the basic fuel injection quantity Tp according to the driving condition cannot follow sufficiently the difference in the correction request according to the difference in the engine driving condition during a period of engine revolutions before the air/fuel mixture ratio learning is greatly advanced, a stepwise difference occurs in the base air/fuel mixture ratio at the time of the engine transient driving condition so as to give an ill effect on the exhaust gas characteristic as appreciated from FIG. 10. There is an industrial demand to perform the self-diagnosing system contained in the fuel injection controlling system itself self-diagnose against the state of controlling the air/fuel mixture ratio and to make it alert the worsening of the exhaust gas characteristic even in a state where the learning of the air/fuel mixture ratio is insufficiently advanced. However, in a state where the correction request is not sufficiently taken into the learning correction coefficient, it is not possible for the self-diagnose system to perform an accurate self diagnose against the air/fuel mixture ratio control state on the basis of the learning value and, therefore, the alert of the worsensing of the exhaust gas characteristic cannot be carried out until the learning on the deviation of the actual air/fuel mixture ratio from the target air/fuel mixture ratio is sufficiently advanced.
As described above, in the self diagnose of the air/fuel mixture control state on the basis of the average value of the learning correction coefficient KBLRC, it is difficult to operate the self diagnose system so as to perform self diagnose having a correlation to the exhaust gas characteristic and it is not posible to perform the accurate self diagnose before the learning is sufficiently advanced.
Consequently, in a case where the worsening of the air/fuel mixture ratio control state wherein the percentage of the harmful components in the exhaust gas exceeds the statutory limit value is sequentially and arbitrarily monitored, the self diagnose system described above cannot be adapted to the monitoring and determination of the worsening of the air/fuel mixture ratio.